Conversely, when the alternating voltage decreases,
three-phase, full-wave rectifier through the
the following events occur:
compensator. The dc output of the rectifier is applied to
the series circuit. This consists of the regulator coil,
1. The regulator operates in the opposite direction.
range-setting resistance, voltage-adjusting rheostat, and
This is because of the pull exerted by the coiled
secondary of the damping transformer. In the following
spring.
description of operation, note that the standby regulator
on the left side of the schematic (fig. 8-2) is not
2. This action shorts out resistance in the exciter
energized or used
field circuit.
When the regulator coil is energized, the magnetic
3. The impulse from the damping transformer
pull on the iron armature is balanced against the
momentarily opposes the decrease in regulator
mechanical pull of the coiled spring.
coil current.
4. This action reduces the extent of the decrease in
THEN
IF
regulator coil current.
The magnetic pull of Silver buttons are separated
5. This then restricts the magnitude of the increase
the armature over- from each other, adding
in exciter field current and armature voltage.
comes the pull of the more resistance in the field
circuit
spring
Damping Transformer
The tension of the Silver buttons are pressed
coiled spring over- t o g e t h e r , c a u s i n g l e s s
The damping transformer is an antihunt device. It
comes the pull of the resistance in the field
consists of two windings placed on the center leg of a
circuit
armature
C-shaped laminated iron core. The primary of this
transformer is connected across the output of the exciter
Thus, the moving arm operates through its travel,
(fig. 8-2). When a change occurs in the exciter voltage,
depending on the direction of motion, to successively
the primary of the damping transformer induces a
open or close the silver buttons. This increases or
voltage in its secondary. The secondary voltage acts on
decreases the resistance in the exciter field. The moving
the regulator coil to dampen the movement of the
arm has a short travel so that all resistance can be
armature. This prevents hunting and excessive changes
inserted or cut out quickly. It can also be varied
in the generator terminal voltage.
gradually. This depends on the required change in
excitation.
The voltage-adjusting rheostat (fig. 8-2) is used to
raise or lower the regulated value of the ac generator
For example, when the alternating voltage rises, the
voltage.
following events occur:
The regulator control switch has three
1. The regulator operates because of the increasing
positions--MANUAL, TEST, and AUTOMATIC.
magnetic pull on the armature.
When the control switch is in the MANUAL
2. This action inserts resistance in the exciter field
position, you can control the ac generator voltage
circuit to reduce the exciter field current and
manually by the exciter field rheostat (fig. 8-2).
armature voltage,
3. The primary of the damping transformer across
When the control switch is in the TEST position (as
the exciter circuit is subjected to this change in
shown), the control element is energized. However, the
current. Through transformer action, a
regulating resistance is shorted out. The current in the
momentary voltage is induced in the secondary
exciter field circuit can be varied by the exciter field
that opposes the increase in regulator coil
rheostat. The operation of the moving arm in the control
current.
element can be observed.
4. This action is a form of negative feedback. It
When the control switch is in the AUTOMATIC
lowers the magnitude of the increase in regulator
position, the generator is under full control of the
coil current.
regulator. The regulator will adjust the voltage to the
value predetermined by the position of the
5. This restricts the magnitude of the decrease in
voltage-adjusting rheostat.
exciter field current and armature voltage.
8-7
